1. Field of the Invention
The present invention relates to a continuous heat treatment furnace for heating a metal strip in a direct fire-heated atmosphere.
2. Description of the Prior Art
In order to prevent surface oxidation at the heating step, it is ordinarily necessary to heat a strip in a reducing atmosphere or an inert gas atmosphere. As the furnace for this heat treatment, there are known an indirect heating furnace using a radiant tube and a direct heating furnace using a direct fire burner. The former furnace is called a radiant tube furnace (RTF), and the latter furnace is called a non-oxidizing furnace (NOF).
The indirect heating method using RTF has been regarded as an excellent means for heating strips because the atmosphere gas can optionally be selected and a high quality product is obtained. However, in order to maintain the air-tightness in the radiant tube, a heat-resistant alloy is ordinarily used as the material for the radiant tube. Therefore, the heating temperature is restricted by the temperature that can be resisted by the tube. Accordingly, this method is defective in that both the heating efficiency and the thermal efficiency are low. Furthermore, the method is defective in that the equipment cost is high.
In contrast, in the direct heating method using NOF, since direct heating is performed by a reducing combustible gas containing unburnt components such as CO and H.sub.2 and having an air ratio lower than 1, the heating efficiency and thermal efficiency are high, though the surface conditions of the treated strip are inferior to some extent to those of the strip treated according to the indirect heating method. Moreover, the equipment cost is low. Accordingly this type of furnace has recently been positively adopted for continuous heat treatment furnaces for strips. There are two modes of application of NOF to the continuous heat treatment for strips. According to one mode, NOF alone is used as the heat treatment furnace. According to the other mode, both NOF and RTF are used in combination so as to effectively utilize the characteristics of both the furnaces, and NOF is arranged in the low-temperature region of conventional RTF.
Vertical NOF as one example of the conventional continuous heat treatment furnaces having the above-mentioned characteristics, will now be described with reference to FIGS. 1 and 2.
Referring to FIGS. 1 and 2, reference numeral 101 represents a furnace wall having a sealing property, a fire resistance, and a heat-insulating property, which defines a furnace; reference numeral 102 represents a strip to be heated, which is passed through the interior of the furnace; reference numeral 103 represents a support and delivery roll disposed in the furnace to support and deliver the strip 102; and reference numeral 104 represents a combustion gas supply device arranged on each of both the end portions of the furnace wall 101. Ordinarily, a plurality of combustion gas supply devices 104 are arranged to both the surfaces of the strip 102 in a staggered manner with respect to the thickness direction. A heat treating burner is ordinarily adopted as the combustion gas supply device 104.
Reference numeral 105 represents a sealing chamber for an inlet or outlet for the strip 102; reference numeral 106 represents a roll chamber for protecting the delivery roll 103 for the strip 102 from the high temperature portion; reference numeral 107 represents a heating chamber for heating the strip 102 by the combustion gas supplied from the combustion gas supply device 104; and reference numeral 108 represents a preheating chamber for preheating the strip 102 by the combustion gas from the heating chamber. Ordinarily, a secondary combustion burner is disposed in the preheating chamber 107 to reburn the combustion gas from the heating chamber 107, which contains unburnt components, whereby the thermal efficiency is enhanced. Reference numeral 109 represents a flue for discharging the combustion gas from the furnace. In FIGS. 1 and 2, a solid line indicates the flow of the combustion gas from the outlet to the inlet in the furnace, and a broken line indicates the advancing direction of the strip.
As shown in FIGS. 1 and 2, in the conventional vertical NOF, heating of the strip 2 is accomplished by the gas radiation heat of the combustion gas charged into the furnace from the combustion gas supply device 104 and the solid radiation heat from the peripheral furnace wall 101. Accordingly, in order to increase the quantity of the transfer heat, it is necessary to increase the thickness of the gas layer and the furnace wall area. However, extreme increase of the sectional area of the furnace is restricted by the equipment cost.
Since the temperature of the combustion gas in the preheating chamber 108 is lower than in the heating chamber 107, the convection heat transfer is dominant. Therefore, from the viewpoint of the heat transfer, it is preferred that the sectional area of the furnace be reduced and the flow rate of the combustion gas be increased. In the conventional NOF, however, since all the combustion gas is caused to flow in a counter-current manner to the strip, in view of the pressure loss in the furnace, it is impossible to extremely reduce the sectional area of the furnace.
Furthermore, in the conventional vertical NOF, since the combustion gas is caused to flow in a counter-current manner to the strip 102, the furnace wall 101 should be arranged on both the sides confronting both the surfaces of the strip 102. Therefore, the furnace should be constructed to have a long shape bent in the vertical direction in such a manner that the strip advancing line is surrounded by the furnace wall 101, as shown in FIGS. 1 and 2. Accordingly, the furnace installation area is increased and the furnace wall area is increased. The conventional vertical NOF thus involves a fundamental problem of increased equipment cost.